Heat treating



Patented Nov. 12, 1940 near TREATING Vincent '1. Malcolm, IndianOrchard, Mass, assignor to The Chapman Valve Manufacturing 00., IndianOrchard, Mass., a corporation of Massachusetts No Drawing. ApplicationApril 6, 1938, Se ial No. 266,421

1 Claim. (01. 148-21 This invention relates to improvements inheat-treating alloy steels and is directed more I particularly to animproved process of heattreating an alloy steel containing chromium,

5 tungsten, molybdenum, and vanadium in two or more combinations and theproduct of that proc- Y ess.

It is a principal object of the-invention to provide a so-called alloybolt steel tobe used in high- IQ pressure, high-temperature servicewhere stability and creep resistance are of prime importance. That is tosay, bolts or the like made from the steel of this invention are adaptedto stand up under high pressures and temperatures as when usedin'bolting flangesin various parts of high pressure and temperatureequipment where leakage at joints must be prevented and whereinstability due to structural changes in the steel because of such highpressures and temperatures.

with consequent low creep resistance, has hith erto resulted in aninability of the bolts to maintain suflicient residual stress and,consequently,

a tight joint.

Tests were made on various types of conventional steels with a newlydeveloped relaxation machine to determine relaxation ofthe materialunder service conditionsand to study the results of complete bolting setup in flanged joints. The result of this studyshowed unusual relaxationso and creep, together with "structural changes in the steel itself;

- As a special feature of this invention, my process produces boltshaving structural constituents superior to those where ordinary heattreatments,

35' such as, heating to critical transformation,

quenching in liquid, and then tempering, are employed. I 1

The ideal structure to attain in steel by means of any heat-treatingprocesses is one of controlled grain without any visible networkformation. In order to insure the absence of network mation by'carryinzout thisportionof the treat- 5o ment at a temperature below thecritical transformation point, In this way the steel may be broughtto auniform heat condition throughout, and be on the verge ofrecrystallization without-dangerof network formation. The soaking periodI then follow by a normal period of heating'abo've the criticaltransformation point to bring about complete recrystallization and grainsize as desired by temperature adjustment in such a way that there isminimum opportunity for segregation to occur. I 5 I then preferably coolthe steel rapidly in air, because I have found that liquid quenchingtends to be too drastic to maintain the desired structure. The drawingor tempering temperature 'is such that maximum strength and ductilityfor 1 the service intended are obtained and in the tempering operationthe steel must be cooled slowly in the furnace to a temperature of below500 F. in order to retain its properties, and to stop secondary networkformation. is

For convenience this method will be termed- "delayed heating normalizingtreatment, and the particular feature of this method which greatlyinfluences the results obtained is the prenormalizing soak at atemperature below the 20 critical transformation point. I have foundthat thorough preheating of this kind seems to overcome the wellrecognized sluggishness in the part these properties'when in service inhigh temperature and pressure installations. Steels treated according tomy process have been tested for-periods of time in temperatures up to1000 F. and found to have retained their structure to the extent that itis directly comparable 40 with that of the material when it was placedin service.

From a standpoint of machineabiiity, the extra hardness is compensatedforby the fact that steels treated according to the processor. thisinvention appreciable network formation and are capable of beingmachined with greater case. That is to say, a of 360. Brinell in mytreatedwill machine as readiiyas of 280 Brinell in the liquid *quenchedsteels. t

I have experimented with various alloy steels in the development of thisprocess, such as one containing, in addition to irbn-,, of course,.from41:0 6% of chromium and either about 5% of from .20-.30% of vanadium.

As a result of this experimenting, I have developed a new alloy whichhas all of the desired characteristics above mentioned. This alloy hasthe following preferred formula, the stated proportions beingapproximate;

Per cent Carbon .35- .55

. Manganese .40 .85 Silicon .45- .75 Chromium .90-1.25 Molybdenum .50-.65 Vanadium .25- .35 Phosphorus, max .04 Sulphur, max .04 iron BalanceThe preferred process of heat treating this type of steel will now bedescribed.

The piece or pieces are first heated slowly in a suitable furnace toabout 1450- F. The heating is delayed at this point by allowing thesteel to soak for about one and one-half hours for each inch ofthickness.

Then the pieces are heated slowly to about 1550" F.'and rapidly to 1750F. Soaking at the rate of hour for each inch of thickness is thencontinued, after which the steel is cooled rapidly in air. I

After the cooling step, I- reheat the pieces to 1250 F. for one andone-half hours for each inch of thickness. This is followed by slowcooling in the furnace to below 500 F. and again cooling in air. s

Bolts processed according to this heat treatment have been used forhightemperature and high pressure service with excellent results. Tests haveshown that they have the following average physical properties:

Charpy impact ft. lbs. keyhole notch 15 Thesesame newly-processed steelstested in temperaturesup to 1000 F. for short periods of time show thefollowing properties:

Tensile strength, lbs. per sq. in 105, 000 Yield point, lbs. per sq. in85, 000 Elongation in '2 inches, percent 2 Reduction of area, percent 62Creep tests of this newly-processed steel made at 1000 F. for a periodof not less than 5000 hours showed a creep limit of 22,000 pounds persquare inch for. 1% creep in 100,000 hours. Relaxation tests based-on60,000 pounds per square inch initial stress at 950 F. showed residualstress of 20,000 pounds per square inch after 1000 hours.

The same steel above described when treated by a conventional process,-that is heated to 1700 F., quenched in oil, drawn to 1250" E, and cooledin 1000 hours.

in air had the following physical properties as contrasted with thoselisted above:

Tensile strength, lbs. per sq. in. 145, 000

Yield point, lbs. per sq. in 126, 000 Elongation in 2 inches, percent17.5 Reduction of area, percent 47 Brinell 310 Charpy impact, ft. lbs 16When this same conventionally treated steel was tested for a short timeunder temperature of 1000 F., it had the following properties ascontrasted with the above-listed properties of my steel shown by thesame test:

Tensile strength, lbs. per sq. in 90, 000 Yield point, lbs. per sq. in65, 000 Elongation in 2 inches, percent 20 Reduction of area 65 Creeptests of this same conventionally-treated steel showed the following, ascontrasted with what was stated above concerning steel treated accordingto this invention: creep limit at 1000 F. for a period of not less than5,000 hours showed a value of 10,000 lbs. per sq. in. for 1% creepRelaxation tests on steel treated according to the prior art based on60,000

lbs. per sq. in. .initial stress at 950 F. showed a residual stress of9,000 lbs per sq. in. in 1000 hours.

- In my investigation a series of test pieces were heated for variousperiods of time at different temperaturesin order to ascertain thestructural changes which take place upon heat treatment.

Complete work on the several steels was carried out at varioustemperatures of from 1350 F. to 2150 F. and, at each temperature, forperiods beginning with 5 minutes, then raising the time in increments of10 minutes until one hour was reached, then in steps of 20 minutes untilthree hours holding time was attained.

After each test for each period of time at each temperature,recrystallization, diffusion, grain refinement, carbide segregation, andnetwork formation were all noted and checked with criticaltransformations. Microesections and hardness determinations were takenof each specimen of each type of steel in each range at various times sothat structures obtained could be critically studied.

Prolonged heating at certain temperatures resulted in a change in-grainsize and a lack of any visible segregation. Above certain temperaturesthe network again appeared and persisted.

Steels containing this network formation are objectionable in that theyshow very little ductility in tension or bending, have low impact value,and are brittle wherefore they are of little value for high-temperatureand/or high-pressure service.

The series of experiments just described showed that two conditionscontrol the structural characteristics of such steels, namelytemperature and time, and that of these, temperature is of primaryimportance.

While I have described the invention in great detail and with respect tothe present preferred form thereof, it is not desired to be limitedthereto since changes and modifications may be made therein withoutdeparting from the spirit and scope of the invention. The inventionmaybe embodiedin other specific form without de parting from theessential characteristics thereof. The present embodiments are thereforeto be considered in all respects as illustrative and not restrictive,the scope of the invention being indicated by the appended-claim ratherthan by the foregoing description, and all changes which come within themeaning and range of equiva lency of the claim are therefore intended tobe embraced therein.

What it is desired to claim and secure by Letters Patent of the UnitedStates is:

The process of heat-treating alloy bolt steels of the pearlitic type ofchromium steel to increase the creep resistance of the steels consistingof, preheating the steel by slowly soaking it at a temperature below itscritical transformation point for a period at the rate of about one and'one-half hours for each inch of thickness of steel, following thesoaking by a period of slow heating at a temperature above its criticaltransformation point for a period at a rate of about one-half hour foreach inch of thickness, rapidly cooling the steel in air, then reheatingthe steel to a temperature below its critical transformation point for aperiod at a rate of about one and one-half hours for each inch ofthickness, and then allowing the steel to cool slowly.

VINCENT T. MALCOLM.

